[media] xc4000: shut up a bogus smatch message

smatch complains about:
	drivers/media/tuners/xc4000.c:1511 xc4000_get_signal() warn: '~value << 3' 524280 can't fit into 65535 'value'

Remove the bogus complain.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>